This invention relates to a multilayer printed wiring board constituted for electric connection between multiple conductor layers by means of a so-called via hole of non-penetrating type, and to a method for manufacutring same.
In the multilayer printed wiring board comprising a lamination of a plurality of conductor layers forming a circuit with insulating layers interposed, the electric connection between the conductor layers is generally attained by forming through holes with a drilling work, and providing a plated metal layer on inner peripheries of the through holes.
In order to establish the wiring at a higher density in recent years, however, there has been started to be widely employed a measure in which the non-penetrating via hole instead of the through hole is formed in the insulating layer, and an electrically conducting substance is provided to the inner periphery of the via hole. For a process of forming this non-penetrating via hole, it has been general to remove through an etching a portion of a metal layer formed on a surface of the insulating layer, at which portion the via hole is to be formed, and then to irradiate a carbon dioxide gas laser onto the thus exposed insulating layer formed by a thermosetting resin or the like so as to provide the via hole in the insulating layer at the portion where the metal layer has been removed.
The method for manufacturing the multilayer printed wiring board with the via hole formed by means of such laser irradiation has been disclosed, in, for example, Japanese Patent Publication No. 4-3676 by Y. Ohsachi et al., according to which an aperture of the same diameter as that of the via hole is formed first through the top metal layer by means of the etching or the like at the position where the via hole is to be made, then the non-penetrating via hole bottomed by an internal conductor layer is formed in the insulating layer with the laser irradiation of the carbon dioxide gas laser or the like at the position where the aperture has been formed, thereafter a metal foil is formed on inner periphery of the via hole by means of a plating, the metal foil is processed through the etching to form a circuit which connects the metal layer and the internal conductor layer through the metal foil, and the multilayer printed wiring board in which a plurality of the metal and conductor layers are electrically connected through the plated metal foils in the via holes thus formed is obtained.
In providing the via hole in the insulating layer with the laser irradiation as has been described, a continuing zone of the inner periphery of the via hole to the bottoming internal conductor layer becomes an angular shape close to be 90xc2x0 in an axially sectioned view of the via hole, so long as the inner periphery of the via hole is of a vetical wall. In providing to such periphery of the via hole the plated metal foil, a thin, electrolessly plated film is first formed through an electroless copper plating or the like plating on the inner periphery of the via hole, and thereafter the metal foil is formed by means of an electro-copper-plating or the like electroplating with a current passed through the plate film. Equipotential surfaces occurring inside the via hole during the electroplating will be parallel to the inner periphery and to the bottom conductor layer of the via hole but will deflect at the continuing zone of the inner periphery of the via hole to the bottom conductor layer, whereby the electric lines of force intersecting at right angles the equipotential surfaces are caused to be coarse at the deflected part of the equipotential surfaces so as to render the current density to be smaller at this part. Also, plating solution is deteriorated in the fluidity at the continuing zone.
Consequently, the plated metal foil formed in the via hole becomes extremely thin at the continuing zone of the inner periphery of the via hole to the bottom conductor layer so as to lower the reliability of the electric connection between the respective conductor layers through the plated metal foil, so that a problem rises in that the connection is broken frequently at such extremely thin part of the metal foil due to a heating upon mounting costituent parts or a generated heat at the mounted parts being used.
In U.S. Pat. No. 4,070,501 to V. Carbine et al., on the other hand, there has been disclosed a provision of the via hole, which is an element in a semiconductor device and is not suggestive to any technical matter characteristic to the multilayer printed wiring board. Japanese Pat. Laid-Open Publication No. 8-279679 of H. Takagi et al. and assigned to the same assignee as the present invention also disclose a formation of the via hole but involves the same problem as in the foregoings.
The present invention has been suggested to overcome the foregoing problem, and an object of the invention is to provide a multilayer printed wiring board high in the reliability of the electric connection between the conductor layers through the plated metal foil in the via hole as well as a method for manufacturing such multilayer printed wiring board.
According to the present invention, the above object can be realized by means of a multilayer printed wiring board in which a plurality of conductor layers are laminated as a whole with insulating resin layers respectively interposed, a non-penetrating via hole is provided in the respective insulating layers as bottomed by each conductor layer exposed, and a plated layer is provided in the via hole for electric connection between the respective conductor layers, wherein the via hole is formed at least at a continuing zone of inner periphery to bottom surface to be of a concave curved surface of a radius of 20 to 100 xcexcm in axially sectioned view of the via hole.